>>> reaction continues tonight following cia director david petraeus' resignation. new details about the e-mails that started it all. >>> good evening, everyone. nbc news is learning that a complaint against petraeus' biographer is what started the investigation. law enforcement officials say petraeus himself did nothing criminal and will not be investigated. darcy spencer's been gathering latest information for us tonight. darcy? >> reporter: jim, this is a complicated story. we're going to make it as simple as we can. sources tell nbc news the fbi recently began investigating complaints from a woman who said she'd been receiving harassing emails. that investigation led to the discovery of emails between petraeus and his biographer that indicated they were having an affair. retired general david petraeus resigns in disgrace from his post as head of the cia after acknowledging what he called unacceptable behavior. he admitted to cheating on his wife of more than 37 years, holley pa trahal holly petraeus. it involved an affair between petraeus and another woman who w